The present invention relates to a surface treated steel material with excellent corrosion resistance, to be used for outer plates, exhaust system members and gasoline tank materials of automobiles, metal construction materials for roofs and walls, public works materials, and home and industrial electrical appliances.
Zn, Znxe2x80x94Al, Alxe2x80x94Si, Sn, Pbxe2x80x94Sn, Cr and Ni platings are among platings used for surface treated steel materials, and because of their excellent corrosion and heat resistance and attractive outer appearance, they have come into wide use for automobile members, construction materials, electrical appliances and container materials. The most commonly used platings are Zn and Znxe2x80x94Al platings. This is because Zn is the only plating metal with a sacrificing protection property to prevent corrosion of exposed iron when the basis iron is exposed. However, the relatively high corrosion rate of the Zn plating itself is a problem. With a Znxe2x80x94Al system, the plating corrosion rate is lower but the sacrificing anti-corrosion action for iron is weaker. Elements that improve the corrosion resistance of the Zn plating itself generally tend to impair the sacrificing anti-corrosion effect, and therefore the corrosion resistance of the plating itself and the anti-corrosion for iron facings are in an antimonious relationship.
Sn platings, Alxe2x80x94Si platings and the like that exhibit excellent corrosion resistance as platings themselves all lack protective action on basis iron exposed in normal environments.
The following is an example of such a plating for protection of exposed basis iron. Specifically, U.S. Pat. No. 3,026,606 describes hot-dip aluminum plating of iron articles in an aluminum bath containing Mg and Si in a stoichiometric relationship in order to obtain Mg2Si up to 25% in the plating layer. According to this patent, it is indeed possible to achieve improvement in corrosion resistance by crystallization of Mg2Si in the aluminum plating layer. However, when Mg is added to the plating bath in excess of 10%, oxidation of the Mg causes highly profuse production of an oxide film on the plating bath, thus making continuous production impossible. Based on investigation by the present inventors, crystallization of Mg2Si in an aluminum plating layer has been found to alter the form from fine to coarse, and thus notably influence the corrosion resistance.
Japanese Examined Patent Publication HEI No. 3-21627 discloses an Alxe2x80x94Znxe2x80x94Sixe2x80x94Mg quaternary alloy plating containing dendritic crystals composed mainly of aluminum and zinc. While it is indeed possible to adequately protect exposed iron with a crystallized plating of dendritic crystals composed mainly of aluminum and zinc, the Zn is added in an amount of 25% or greater, which impairs the corrosion resistance of the plating itself.
It is an object of the present invention to provide plated steel sheets exhibiting both high corrosion resistance of the plating itself and a protective effect on exposed basis iron in a continuous production process, which had been considered impossible to simultaneously achieve
According to the present invention there has been successfully achieved a surface treated steel material based on a completely different concept unlike the conventional concept of protection of basis iron by sacrificing anti-corrosion action and plating base metal corrosion product coating action.
It is known in the prior art that corrosion resistance is improved by the coating action of stabilized corrosion products obtained by addition of Mg to Zn-based platings. As a result of repeated diligent study on ways of exhibiting the corrosion inhibitor effect of Mg even in Al-based and Si-based platings as well as Zn-based platings, the present inventors have completed the present invention upon finding that by including Mg in the plating as a water-soluble intermetallic compound (Mg2Sn or Mg2Si) in the form of massive bodies of consistent sizes, the intermetallic compounds elute out from the plating coat by contact with water in a corroding environment and form an anti-corrosion film composed mainly of Mg hydroxide, thus allowing a notable improvement in the plating corrosion resistance.
As a result of searching for elements with anti-corrosion action in addition to Mg, it was found that elements of the same Group as Mg, Group IIa of the Periodic Table (alkaline earth metals), exhibit an anti-corrosion corrosion effect. The anti-corrosion effect is most with Mg and Ca among the alkaline earth metals.
Intermetallic compounds are generally considered to be poorly soluble in water, but combinations of elements with a large difference in potential negativity are water-soluble. Much research has been conducted on electronegativities of elements, but here we will rely on the values from the research by Pauling. Intermetallic compounds composed of elements having a minimum/maximum electronegativity ratio of 0.73 or lower are soluble in water. The electronegativities of alkaline earth metals are generally low and intermetallic compounds containing these elements are readily soluble in water, but upon examining solubilities in water, it was found that intermetallic compounds composed of alkaline earth metals and Group IVb elements have considerably high solubilities in water. The most preferred Group IVb elements that form intermetallic compounds with Mg and Ca are Si and Sn, because of their electronegativities.